As per the title, in looking into why a device detection VCL wouldn't work correctly with Fastly I discovered this is because the Fastly module doesn't prevent Drupal core from maintaining an internal page cache. Therefore you end up essentially with two page caches, the internal one and the Fastly one.
I believe this is also responsible for other oddness we've been observing where editors have been complaining that their changes aren't appearing immediately on the sites (even with expires sending the correct purge requests).
Patch to follow, and due to time restraints it's all shamelessly ripped off from the latest dev Varnish release.
Closed: outdated
2.0
Code
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.